29Si MAS NMR is a well-established tool for quantitative characterization of Si-Al distribution in aluminosilicates. In framework aluminosilicates, all tetrahedrally coordinated Si/Al (T) are linked via bridging oxygens to four other Si/Al (Q 4). Each crystallographically unique T site is commonly assumed to give at most five 29Si NMR peaks with chemical shift spacing around 5 ppm, corresponding to five Si units each linked to nSi and (4 - n)Al (n = 0-4), abbreviated as Si(nSi) hereafter. Here we report a detailed one- and two-dimensional (2D) 29Si NMR study on K-cymrite (KAlSi 3O 8·H 2O), which possesses a double-layered structure with all Si/Al distributed in one crystallographically unique T site (Q 4). Contrary to general belief, more than five 29Si MAS NMR peaks were resolved. These peaks are attributable, via 2D J-coupling mediated experiments, to Si(nSi)(n = 0-4), with two peaks each (∼2 ppm spacing) for n = 1-3. The latter corresponds to different permutations of the nSi(4 - n)Al over neighboring T sites of two distinct Si-O-T angles (139 and 180°). This suggests for the first time that the 29Si chemical shift varies with Si-O-Si and Si-O-Al angles differently. Si-Al exchange over neighboring T sites of unequal Si-O-T angles is another possible cause for multiple 29Si NMR peaks.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films